278 Broadband Balanced Microstrip Antenna Fed by a Waveguide Coupler R. Gotfrid*, Z. Luvitzky*, H. Matzner* and E. Levine** * HIT, Holon Institute of Technology Department of Communication Engineering, 52 Golomb st, Holon 58702, Israel ** Afeka Academic College of Engineering Tel Aviv, 38 Mivtza Kadesh, Tel Aviv 69107, Israel Abstract- A balanced microstrip antenna fed by H- shaped aperture, made on the broad wall of a rectangular waveguide, with an external waveguide coupler is presented. The measured bandwidth of the antenna is 35% for VSWR = 2 and the gain is 7-8 dbi. In addition, another version of the antenna with an integrated flat, waveguide coupler has been simulated, showing same bandwidth with higher gain (9-10 dbi). II. GEOMETRY. A. External Hybrid Coupler An overall view of the proposed antenna with an external feed structure is shown in figure 1. Index Terms - balanced microstrip antenna, waveguide hybrid coupler I. INTRODUCTION Broadband microstrip antennas attract a lot of attention in recent years [1]-[2]. Proposed structures include multiple layers, tapered feeds and unique excitation techniques such as the H- shaped aperture [3]. Balanced feed antennas are less sensitive to environmental effects such as avoiding the degradation in handset antenna performance when held by users [4], and in RFID tags [5]. In addition, the radiation pattern of a balanced antennas is more symmetric in comparison to single-ended fed antennas. In this work we propose a broadband, balanced microstrip antenna fed by H-shaped aperture located on the broad wall of a rectangular waveguide, while the waveguides are fed by an external 180 o waveguide hybrid. Also we suggest a similar antenna with an integrated flat 180 o waveguide hybrid coupler. The structure of the paper is as follows: section II describes the geometry of the antenna and the feeds. Section III shows simulation results and section IV shows measured results. Section V concludes the work. Fig. 1 View of the antenna and its feed The antenna is fed by two out-of-phase SMA connectors. The two patches are printed on the lower side of the substrate. The metallic vertical wall supports the substrate and also reduces the mutual coupling between the two ports. The two waveguide sections are separated by an inner wall. Figure 2 shows the inner view of the waveguide
279 Fig, 2 An inner view of the waveguide. The size of each radiating element is 18.5 x 32 mm. The substrate used is Taconic RF35 (dielectric constant 3.5) of thickness 1.52 mm. The height of the radiating elements above ground is 17 mm and the distance between the elements is 50 mm. The inner size of the waveguide is 61 x 13 mm and the thickness of the waveguide walls is 2 mm. The distance between the SMA connector to the walls of the waveguide is 16 mm. The thickness of the wall above the waveguide is 3.5 mm and its width is 21.5 mm. The transition elements are U-shaped. The external waveguide hybrid, based on a thick metallic cylinder inside a rectangular waveguide, is shown in figures 3 and 4. Fig. 4 An inner view of the external waveguide hybrid. The inner dimensions of the waveguide are 62 x 34 mm. The diameter of the thick cylinder is 12 mm, its height is 18 mm and its distance from the waveguide wall is 22 mm. The diameter of the thin cylinder is 2.5 mm, its height is 11.5 mm and its distance from the waveguide wall is 14 mm. The top of the thin cylinder is located 4.5 mm below the upper wall of the waveguide. B. Integrated Hybrid Coupler The antenna with the integrated hybrid coupler is shown in figures 5 and 6. Fig. 3 An outer view of the external waveguide hybrid Fig. 5 Overall view of the antenna with an integrated hybrid coupler.
280 III. SIMULATIONS A. External Hybrid Coupler The simulated return loss of the antenna is presented in figure 7, showing a bandwidth of 35% for VSWR = 2. The coupling between the two ports is shown in figure 8, showing isolation values between -17 db to -26 db at the relevant bandwidth. Fig. 6 The aperture of the antenna with the integrated hybrid coupler. The size of each radiating elements is 21 x 27 mm, printed on the same Taconic RF35 substrate mm. The height of the elements above ground is 7 mm, the thickness of the upper wall supporting the substrate is 4 mm and its width is 10 mm. The distance between the element centers is 41 mm. The inner size of the hybrid waveguide is 57 x 8 mm and the thickness of the waveguide is 2 mm. Near the SMA input connector there are two walls with thickness 1.7 mm, and in the middle of the walls there is a round hole with radius 2 mm. An horizontal wire with length 21 mm and radius 1 mm touches two vertical plates. The right vertical plate touches the top of the waveguide, and the left vertical plate touches the bottom of the waveguide. The height of a plate is 7 mm, the width is 10 mm and thickness is 1 mm. Fig. 7 Simulated return loss of the antenna with the external hybrid coupler. Fig. 8 Simulated coupling between two antenna ports with the external hybrid coupler.
281 The S parameters of the hybrid (S11, S12 and S21) are shown in figure 9. The amplitude difference between the two ports of the hybrid is lower than 0.1 db, and the phase error is lower than 1 o. The operating frequency of the hybrid is between 2.9 to 4.1 GHz or bandwidth of 37% for VSWR = 2. (b) H-plane radiation cut at 3 GHz. Fig. 10 Simulated radiation patterns at 3 GHz. Fig. 9 Scattering parameters of the external hybrid coupler. Radiation patterns in the E-plane and the H-plane at 3 GHz are presented in figure 10. Radiation patterns at 4 GHz are shown in figure 11. The beamwidth details are written on the figures. The gain is 7-8 dbi within the entire band. (a) E-plane radiation cut at 4 GHz (b) H-plane radiation cut at 4 GHz. Fig. 11 Simulated radiation patterns at 4 GHz. (a) E-plane radiation cut at 3 GHz.
282 B. Integrated Hybrid Coupler The simulated return loss of the antenna with the integrated hybrid coupler is presented in figure 12. The S parameters of the hybrid (S11, S12 and S21) are shown in figure 13. The amplitude difference between the two ports of the hybrid is lower than 0.1 db, and the phase error is lower than 1 o. The operating frequency band of the hybrid is between 2.6 to 4.2 GHz showing bandwidth of 47% for VSWR = 2. 14 and polar radiation cuts at 4 GHz are shown in figure 15. The gain is 8.3-10.4 dbi at the entire band which is higher than with the external coupler. Some squint is noticed in the E-plane. (a) E-plane radiation cut at 3 GHz. Fig. 12. Return loss of the antenna with the integrated hybrid. (b) H-plane radiation cut at 3 GHz. Fig. 14 Simulated polar radiation patterns at 3 GHz. Fig. 13 Scattering parameters of the internal hybrid coupler. The frequency band of the antenna is between 3 and 4 GHz (bandwidth 26% at VSWR = 2) which is lower than with the external coupler. Polar radiation patterns at 3 GHz are shown in figure
283 Figure 17 shows the measured return loss and figure 18 shows the isolation between the two ports. The measured bandwidth of the antenna is close to the simulations, but the center frequency is higher It is shown that the measured isolation between the two ports of the antenna is quite similar to the simulated result. (a) E-plane radiation cut at 4 GHz. (b) H -plane radiation cut at 4 GHz. Fig. 15 Simulated polar radiation patterns at 4 GHz. Fig. 17 Measured return loss of the antenna with the external hybrid. IV. MEASUREMENTS A picture of a laboratory prototype of the antenna and the external hybrid coupler is shown in figure 16. Fig. 18 Measured coupling betweeb the two ports of the antenna. Fig. 16. A picture of the antenna and the hybrid.
284 The measured H-plane and E-plane radiation patterns are shown in figure 19 in several frequencies. Beamwidths are ranging between 50 to 80 as expected and the gain is 7-8 dbi. V. CONCLUSION A wideband, balanced microstrip antenna has been presented. Two cases have been discussed: an antenna with an external 180 o waveguide hybrid coupler and an antenna with integral waveguide hybrid coupler. Simulations and measurements are in good agreement. Some additional effort is required in order to enlarge the bandwidth of the integral coupler ant to cancel the minor squint of the beam REFERENCES (a) Measured H-plane pattern of the antenna. [1] Z. Ning Chen and M.Y.W. Chia, Broadband Planar Antennas: Design and Applications, Wiley, 2005. [2] K. L. Wong, Compact and Broadband Microstrip Antennas, Wiley, 2002. [3] S. C. Gao, L. W. Li, M. S. Leong and T. S. Yeo, "Wide- Band Microstrip Antenna With an H-Shaped Coupling Aperture", IEEE Transactions on Vehicular Technology, Vol. 51, No. 1, pp. 17-27, 2002. [4] A. G. Alhaddad, R. A. Abs-Alhameed, D. Zhou, C. H. See, E. A. Elkhazmi, and P. S. Excell, "Compact Dual-band Balanced Handset Antenna for WLAN Application", PIERS Online, Vol. 6, No. 1, 2010. [5] J. Shen, F. Z. Shen, B. Lv, X. Wang, F. Zhou, J. T. Huangfu, and Ran, L. X., "A near-field small balanced antenna for Micro-RFID system utilized in mobile phones", International Communication Conference on Wireless Mobile and Computing, pp. 598 601, 2009. (b) Measured E-plane pattern of the antenna. Fig. 19 Measured radiation Patterns in several frequencies.